Curable compositions comprising aqueous solutions of water-soluble silicate and water-soluble zincate latent insolubilizers

ABSTRACT

Curable one-package compositions which form products having excellent water resistance as well as other desirable properties are prepared by forming aqueous solutions of water-soluble silicates and water-soluble zincate latent insolubilizers selected from the group consisting of alkali metal or quaternary ammonium zincates having dispersed therein certain inorganic pigments, preferably TiO 2 . These curable compositions may be employed in a number of applications such as, for example, coatings, castings and moldings, but they are particularly useful in protective and decorative applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.740,984, filed Nov. 11, 1976, now abandoned.

BACKGROUND OF THE INVENTION

During the past several years, environmental pollution concerns have ledthose active in the coatings, castings and moldings arts to developvarious curable compositions in which organic solvents derived frompetroleum have been eliminated or at least substantially minimized.Thus, compositions such as water-based compositions and high solidscompositions have been extensively investigated. Even more recently thepetroleum embargo and the resultant high cost and scarcity of petroleumby-products has engendered interest in the development of curablecompositions which are composed entirely of, or at least substantiallyof, inorganic components.

The present invention relates to a curable composition composedessentially of inorganic constituents. More particularly, the presentinvention relates to curable compositions comprising aqueous solutionsof water-soluble silicates and water-soluble zincate latentinsolubilizers, having dispersed therein an inorganic pigment,preferrably TiO₂.

A number of water-soluble silicate compounds are known in the art. Suchcompounds include sodium silicate, potassium silicate, lithium silicateand quaternary ammonium silicates which readily dissolve in water toform solutions therewith. Inasmuch as the pH of these solutions isnormally higher than about 10.5, the silica will remain soluble therein.However, when these water-soluble silicates are blended withinsolubilizing agents (i.e., crosslinkers) quite often the resultantsolution is unstable. Thus, for example, the combination ofwater-soluble silicates with metal oxides of the form M_(x) O_(y), suchas cadmium oxide, copper oxide, calcium oxide, lead oxide and the likehas produced unstable systems in that these materials ordinarily causeprecipitation or gelation. In addition to problems of compositionstability, compositions formed from water-soluble silicates andwater-soluble insolubilizers have in the past proven to be extremelysensitive to water. Thus, such compositions have heretofore beendeficient in wet abrasion resistance.

Accordingly, a curable composition containing a water-soluble inorganicsilicate and a water-soluble insolubilizer of crosslinker which producesproducts having excellent resistance to water as well as other desirableproperties would be of considerable advantage.

SUMMARY OF THE INVENTION

The present invention relates to pigmented, curable compositions formedfrom aqueous solutions of water-soluble silicates and water-solublezincate latent insolubilizers elected from the group consisting ofalkali metal or quaternary ammonium zincates.

Pigmented curable compositions based on aqueous solutions of thesewater-soluble silicates and water-soluble zincate latent insolubilizersexhibit excellent resistance to water as well as having excelleentdurability and resistance to burning. Moreover, the curable compositionsof the invention have the additional advantage that they can be cured bydrying in air at ambient temperature or by the use of relatively lowbaking temperatures. The removal of water by air drying or baking causesthe zincate latent insolubilizer to become reactive and to crosslink thewater-soluble silicate, thereby producing a solid product.

It is noted that the pigmented systems of the invention have propertiessuperior to the corresponding unpigmented systems.

DESCRIPTION OF THE INVENTION

This invention is concerned with the discovery of curable compositionscomposed essentially or substantially of inorganic constituents. Thecurable compositions of the invention comprise aqueous solutions ofwater-soluble silicates and water-soluble zincate latent insolubilizers,having dispersed therein an inorganic pigment, preferrably TiO₂.

Water-soluble silicates which may be employed in the compositions hereininclude sodium silicate, potassium silicate, lithium silicate,quaternary ammonium silicates and the like. A preferred water-solublesilicate is potassium silicate.

Water-soluble zincate latent insolubilizers which are employed in thecompositions of the invention are alkali metal or quaternary ammoniumzincates.

Illustrative of such zincate latent insolubilizers which can be employedare alkali metal zincates such as, for example, lithium zincate,potassium zincate, sodium zincate and the like.

The amount of zincate latent insolubilizers included in the compositionsherein can very considerably depending upon desired properties. However,in general, the amount of zincate latent insolubilizer employed is anamount sufficient to enhance wet abrasion and water resistance of thecured composition but which is insufficient to cause package instabilityin the uncured composition. A stable package is defined as a packagewhich when aged for 24 hours at 160° F. has not formed an irreversiblegel and which still provides a wet abrasion resistant coating. Preferredcompositions are stable at 160° F. for at least about two weeks. Wetabrasion resistance is evaluated by soaking a cured, coated glass panel,drawn with a 3 mil drawbar, in water (temperature 70° F. + 5° F.) forone hour and then rubbing the surface of the cured coating with a watersoaked cloth and determining the number of back and forth finger rubs(double finger rubs) needed to abrade the coating surface and expose thesubstrate. 50 double finger rubs are considered as the minimumacceptable abrasion resistance. Preferred coatings pass 100 doublefinger rubs, while the most preferred coatings pass 200 double fingerrubs without complete abrasion. In most instances, amounts of zincatelatent insolubilizers ranging from 0.1 percent to 5.0 percent,preferably 0.25 percent to 2 percent by weight, based upon the combinedweight of the water-soluble silicate and zincate latent insolubilizermay be employed.

The reaction between the water-soluble silicate (i.e., alkali silicate)and zincate latent insolubilizer is not known with certitude, however,it is theorized that while water is present the zincate and the silicatedo not interact. However, upon drying (e.g., air drying or baking) theequilibria shift and the zincate then crosslinks the silicate to form aninsoluble silicate composition having excellent properties such as stainresistance, flame and heat resistance and water resistance.

In addition to the water-soluble silicate and metallate latentinsolubilizer, the curable compositions of the invention contain aninorganic pigment selected from the group consisting of titaniumdioxide; zirconium dioxide; alumina, preferrably hydrated alumina or tinoxide. The amount of this pigment should be an amount which provides aleast about a 2.5% pigment volume concentration (PVC). The maximum ofthis pigment which can be employed is that amount which together withother "inert solid" i.e. solids other than the solids contributed by thesoluble silicate-latent insolubilizer, is the "critical PVC" i.e. theconcentration of material above which the amount of binder present isinsufficient to provide a coherent, abrasion resistant film. Obviouslythis upper limit will vary with the precise materials employed, but isreadily determinable by minimal experimentation. In most casesinvestigated, this upper limit appears to be at a PVC of about 70percent.

In the preferred embodiment the inorganic pigment is titanium dioxide,preferrably having a surface coating of Al₂ O₃ /SiO₂. Such coatedtitanium dioxide pigments are known in the art.

In addition to the above essential components the curable compositionsof the invention may contain certain other additives usually employed incoating compositions including dyes, coloring pigments, inorganicfillers or extenders, flow control agents, surfactants and the like. Asknown in the art many of these coating additives may be partially orcompletely organic in nature, if desired.

Another unusual aspect of the compositions of the invention is thediscovery that the addition of phosphate type surfactants to thesecompositions improves adhesion without impairing water resistance. Thus,for example, phosphate type surfactants or wetting agents such aspotassium tripolyphosphate, sodium tripolyphosphate and organophosphates such as PK-90, available commercially from the DexterChemical Corporation, may be employed.

In addition it is often advantageous to include in the composition athickener. Of particular value are water-soluble thickeners which aresolubilized through carboxyl groups. Illustrative of such materials aresodium polyacrylate, potassium polymethacrylate, sodium salts of styrenemaleic anhydride copolymers, sodium carboxymethyl cellulose, potassiumpolyacrylate copolymers and the like. Also suitable are inorganicthickeners such as clays and the like.

As mentioned above, the curable compositions of the invention can beemployed in various applications such as for coatings, castings ormoldings. The compositions are particularly useful in coatings over avariety of substrates such as wood, metals, glass, wallboard and thelike.

The curable compositions herein, when used as coatings, can be appliedby any conventional method, including brushing, dipping, rolling, flowcoating, roll coating, spraying and the like. Conventional spraytechniques and equipment can be utilized.

The curable compositions of the invention may be cured by baking atmoderate or elevated temperatures if desired. However, one advantageousproperty of these curable compositions is that they can be cured bydrying them in air at ambient temperature without the use of heat. Aswill be evident, ambient temperature curable compositions are ofconsiderable advantage in that they provide for significant energysavings in comparison to compositions which require elevatedtemperatures for curing.

The following examples are submitted for the purpose of furtherillustrating the nature of the present invention and should not beconstrued as a limitation on the scope thereof. All parts andpercentages in the example and throughout the specification are byweight unless otherwise indicated.

In systems involving insolubilization of soluble silicates, variation inhumidity during cure can lead to variability in results. Therefore inthe following examples, comparisons of examples within each testgrouping are valid while cross comparisons involving separate groupingscured under differing conditions (i.e. humidity) may not be valid.

EXAMPLE 1

A curable coating composition was prepared using conventional paintmixing techniques and equipment by admixing 231.0 grams of an aqueouspotassium silicate solution having an SiO₂ :K₂ O mole ratio of 3.3:1 anda solids content of 38.8 percent by weight, 1.8 grams of zinc chloride,2.8 grams of potassium hydroxide, 180 grams of water, 105.0 grams oftitanium dioxide, 72.5 grams of mica and 10.0 grams of attagil 40.

The curable composition resulting from the above procedure was drawndown on several sets of glass panels using a 3 mil Bird Bar. One set ofcoated glass panels was cured by air drying at ambient temperature forone day, a second set of coated glass panels was cured by air drying atambient temperature for 6 days.

The cured coatings were then evaluated for wet abrasion resistance bysoaking the coated glass panels in water (temperature 70° F. ± 5° F.)for one hour and then rubbing the surface of the cured coating with awater soaked cloth. The web abrasion resistance was evaluated bydetermining the number of back and forth finger rubs (recorded as doublefinger rubs) needed to abrade the coating surface.

The coating cured by air drying at ambient temperature for two daysabraded after 3 double finger rubs while the coating cured by air dryingat ambient temperature for 6 days did not abrade after 200 double fingerrubs.

EXAMPLES 2-9

Coating compositions were prepared by formulating the followingingredients using ordinary coating formulation and grinding techniques:

    __________________________________________________________________________               Parts by Weight                                                    Ingredients                                                                              Ex. No.                                                                            2  3  4  5  6  7  8  9                                        __________________________________________________________________________    Aqueous potassium                                                             silicate solution.sup.1                                                                       350                                                                              350                                                                              350                                                                              350                                                                              350                                                                              350                                                                              350                                                                              350                                      water           73 73 73 73 73 73 73 73                                       ZnCl.sub.2      1.8                                                                              1.2                                                                              0.6                                                                              2.4                                                                              3.0                                                                              0.3                                                                              0.6                                                                              0.6                                      KOH             2.8                                                                              2.1                                                                              1.4                                                                              3.5                                                                              4.2                                                                              0.7                                                                              1.4                                                                              1.4                                      organic phosphate                                                             wetting agent.sup.2                                                                           0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                                                              0.6                                      silicone antifoam                                                             agent (SWS 214) 1.5                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.5                                                                              1.5                                      TiO.sub.2 (CLNC grade)                                                                        105                                                                              105                                                                              105                                                                              105                                                                              105                                                                              105                                                                              -- --                                       alumina         -- -- -- -- -- -- 82 --                                       silica          -- -- -- -- -- -- -- 68                                       mica            90 90 90 90 90 90 90 90                                       magnesium aluminum                                                            silicate (Attagel 40)                                                                         10 10 10 10 10 10 10 10                                       __________________________________________________________________________     .sup.1 A solution having a SiO.sub.2 :K.sub.2 O mole ratio of 3.93:1 and      solids content of 29.1 percent by weight.                                     .sup.2 PK-90 - an organic phosphate available from Dexter Chemical            Company.                                                                 

A series of coated glass panels were prepared by drawing down each ofthe above coatings on glass panels using a 3 mil Bird Bar. After airingat ambient temperature for five days, the cured coatings were evaluatedfor wet abrasion resistance as in Example 1.

    ______________________________________                                        Example No.   No. of Double Finger Rubs                                       ______________________________________                                        2             >200                                                            3               67                                                            4             center of film >200 edges 40                                    5             >200                                                            6             >200                                                            7             >200                                                            8             >200                                                            9             >200                                                            ______________________________________                                    

After 3 days at 160° F. no hard gel was noted in 1/2 pint packages,Example No.s 2, 5 and 6 had a heavy visicosity when stirred.

According to the provisions of the Patent Statutes, there are describedabove the invention and what are now considered to be its bestembodiments. However, within the scope of the appended claims, it is tobe understood that the invention can be practiced otherwise than asspecifically described.

What is claimed is:
 1. A curable composition comprising an aqueoussolution of a water-soluble silicate and a water-soluble latentinsolubilizer selected from the group consisting of alkali metal orquaternary ammonium zincates, having dispersed therein an inorganicpigment selected from the group consisting of titanium dioxide,zirconium dioxide, alumina and tin oxide in an amount which provides anat least about 2.5 percent pigment volume concentration, and wherein theamount of said insolubilizer included in the composition is sufficientto enhance the wet abrasion resistance of the cured coating, butinsufficient to cause rapid package instability in the uncured state. 2.The curable composition of claim 1 wherein said latent insolubilizer isan alkali metal zincate.
 3. The curable composition of claim 1 furthercontaining a thickener.
 4. The curable composition of claim 3 whereinsaid thickener is a water-soluble thickener which has been renderedwater-soluble by reaction of carboxyl groups contained in said thickenerwith base.
 5. The curable composition of claim 1 further containingphosphate type surfactants and wetting agents.
 6. The curablecomposition of claim 3 further containing phosphate type surfactants andwetting agents.
 7. The curable composition of claim 1 wherein saidwater-soluble silicates are selected from the group consisting oflithium silicate, potassium silicate, sodium silicate or quaternaryammonium silicates.
 8. The curable composition of claim 12 wherein theamount of said latent insolubilizer employed in said composition is fromabout 0.25 to about 2.0 percent by weight based upon the combined weightof the water-soluble silicate and latent insolubilizer.
 9. The curablecomposition as in claim 1 where the inorganic pigment comprises titaniumdioxide.
 10. A method which comprises coating a substrate with thecomposition of claim 1 to provide a decorative or protective coating.11. The curable composition of claim 1 wherein the amount of said latentinsolubilizer employed in such composition is from about 0.1 to about 5percent by weight based upon the combined weight of the water-solublesilicate and latent insolubilizer.
 12. The curable composition of claim11 wherein the pigment volume concentration is less than about 70percent.